Electric arc stabilization in electric arc melting using carbon electrodes



US. Cl. 313-657 8 Claims ABSTRACT OF THE DISCLOSURE An arc meltingmethod for use in the melting of metal and in the production of metalfrom metal-containing ore and a reducing agent, in an arc furnace. Themethod stabilizes an are formed between the metal in the furnace and atleast one carbon electrode by providing the electrode with a coreincluding a substance which, under the action of the arc, ionises toincrease the electrical conductivity of the arc. The substance includedin the core of the carbon electrode may be lithium, barium, calcium,magnesium, chromium, manganese, silicon, potassium, sodium, rubidium, orcaesium or compounds such as oxides, carbonates, or silicates of suchelements. Such substance is added in particular form and bonded with abinder such as tar, pitch, or a paste or anthracite, tar and pitch.

This invention relates to the stabilisation of electric arcs and to theprovision of electric arc apparatus, and more particularly electric arcfurnace apparatus capable of conferring improved stability to the arc.

In larger direct arc melting furnaces, by which is meant furnaces inwhich the arc is struck between a carbon electrode and the metal to bemelted, one problem is the heavy surging which occurs when making theare since a virtual short circuiting condition is present. The extent ofthe surging is kept within limits by having a reactance in the circuit,but where larger power inputs are involved, such as ten to fortythousand kva. the surging is sufiicient to cause flicker on the supplysystem.

It is an object of the present invention to reduce such surging effectsand to obtain other advantages such as improved power efliciency andlower electrode consumption.

According to the present invention there is provided a method for thestabilisation of an electric are which comprises introducing into thearc a substance which under the action of the arc generates, or isconverted into, a vapour or median which increases the electricalconducitvity of the arc.

The introduction of the said substance may be eiiected by any of anumber of different methods. Thus the substance may be included in or onone or more arc electrodes or the substance may be introduced into theare as gasborne particles, e.g. by spraying the substance into the arc,e.g. through a hollow electrode or introducing it via a lance, 'or thesubstance may be provided as a solid rod which is continuously fed intothe arc at a rate balancing its rate of consumption in the arc. Or aplurality of the foregoing methods may be used in combination in orderto ensure a sufficient supply of the substance to create ionisedparticles thereof in the arc and thus to increase the conductivity inthe path of the arc.

The invention further includes electric arc apparatus and moreparticularly electric arc furnace apparatus wherein at least one of thearc electrodes carries or embodies a substance as aforesaid and/ orwherein means are ice provided to introduce a said substance into theare from an external source.

The invention will be described with particular reference to the casewhere the said substance is included in, or carried on, at least one ofthe arc electrodes.

The invention is of particular value where the arc electrode orelectrodes are formed essentially of carbon (e.g. graphite).

In selecting the substance for inclusion in the electrode or electrodesregard should be had to the purpose to which the arc is being put, sothat materials deleterious to that purpose are avoided. Specifically itis desirable to select a material of relatively low cost, which isreadily worked (and, if a metal compound, is readily reduced) and whichprovides adequate vapour or other medium of suflicient conductivity.Where an electric arc furnace apparatus used for melting metal isconcerned, the selected substance should be one which has no contaminantor other deleterious effect on the metal to be melted or on the furnacestructure. Within these desiderata a wide range of substances isavailable for use. Generally it is convenient to use metals per se andthese may be included from relatively low melting metals e.g. lithium,metals of medium melting point e.g. barium, calcium and magnesium, andmetals of high melting point, e.g. chromium and manganese. Certainnonmetals, e.g. silicon, may also be employed.

Instead of using the metal per se a compound of the metal, or forexample of silicon, may be employed with reliance on other ingredientspresent in the electrode to cause reduction of the said compound. Thusmetal oxides, carbonates and silicates are very useful. Where, as willbe most common the electrode or electrodes are formed essentially ofcarbon this may itself serve as the reducing agent. However otherreducing agents may be employed, e.g. for the reduction of manganesedioxide, silicon or aluminium may be included.

It will be understood that several substances which afford theconductive vapour or medium may be used in admixture. A mixture ofsubstances of particular value is provided by burnt dolomite whichcomprises calcium and magnesium oxides.

It has been found that the inclusion of compounds of the alkali metals,namely lithium, sodium, potassium, rubidium and caesium, is particularlyeffective in lengthening the arc and improving its stability. It isthought that this is due to their low ionisation potential, i.e. theirreadiness to ionise in the are thereby increasing the electricalconductivity.

On the other hand, compounds of sodium and lithium in particular arevery readily vapor-ised and their use is less preferred since theirpresence in any substantial concentration in a furnace tends to flux therefractory materials of which the furnace lining is made. Potassium isnot so detrimental in this respect and its compounds have been used verysuccessfully for incorporation in graphite electrodes, as will bedemonstrated in the examples which follow. Rubidium and caesium areeffective but their use is difiicult to justify in view of their highcost.

The substance relied on for the provision of the conductive vapour maybe included in the electrode or electrodes in any convenient way. Thusit may be included as an ingredient of the composition of which theelectrode is made or it may be provided as a sleeve or core to theelectrode. Generally it will be present in particulate form and bondedtogether with a binder which may be for example, tar or pitch, or apaste made from anthracite, tar and pitch.

Particular advantages have been found to accrue from the use of amixture of oxide in an excess of carbon paste, e. g. a paste ofanthracite, tar and pitch. When this is baked to form the electrode theproduct is found to have a cellular or porous structure. When theelectrode thus formed is put to use and so becomes heated the dispersedoxide is reduced to metal which is retained in the pores of theelectrode until, by the heat generated in the are, it is vapourised. Theavailability of the metal for the purpose of the present invention isthus enhanced. An analogous result is obtained if the oxide is dispersedin a coking coal.

For most purposes the substance of choice has been found to be apotassium salt, e.g. potassium carbonate alone or with other substances,in a carbon electrode.

The optimum selection of materials and mode of introduction thereof isdetermined largely by the use to which the arc is to be put. Forexample:

(a) When melting zinc in indirect arc furnaces, or zinc containingalloys, zinc oxide or zinc drosses are preferably used.

(b) When melting steel in large arc furnaces it is preferred to usemetals which will not substantially affect the quality of the resultantsteel, preferably silicon and manganese, or oxides thereof. If themetals themselves are used they are preferably in the form offerrosilicon or ferromanganese. When the oxides are used, a paste of theoxides is preferably compounded as indicated above.

By the method of the invention not only is the surging referred to abovereduced or eliminated but flicker and the creation of harmonics in thesupply system are also reduced or eliminated.

The steadier are conditions enable more power to be developed in a giventime and tend to improve the load factor. This higher load factor hasthe effect of increasing the melting rate and the output of the arcfurnace. Moreover, the noise of the arc is considerably reduced, whichis an important advantage, especially for indirect arc furnaces.

Further, the electrode control system need not be so sensitive orresponsive and can, therefore, be of a more simple design.

The following examples will serve to illustrate the invention.

Example I A test was carried out on a 150 kva. single phase indirect arcfurnace, i.e. a furnace in which the arc is struck between two carbonelectrodes. The electrodes in this case were of graphite and were 4inches in diameter. The electrodes were bored with a concentric hole 1%inch diameter which was packed with a core consisting of 60% anthracite,20% pitch with a melting point of 50 C., 16% powdered glass and 4%potassium carbonate. These materials were premixed and rammed into thehole in the electrode which had previously been scored to provide amechanical key for the core and was heated to 40-50 C.

The arc conditions were observed using standard graphite electrodes whenthe furnace was used for melting 750 lb. charges of phosphor-copper.Under these conditions the furnace load fluctuated between 105 and 145kva., the fluctuations being most pronounced at the start of the meltwhen the charge was cold. The wave form of the electric supply wasextremely irregular and the noise emitted by the arc was veryconsiderable.

The test was repeated with the cored electrodes made up as described,but with the same phosphor-copper charge. A remarkable improvement Wasimmediately apparent; the supply wave form was practically sinusoidal,the load fluctuations were reduced within the limits of 140 to 150 kva.and the noise emitted by the arc was diminished to a very low level. Themelting time was reduced by 15% and the energy consumed in melting thecharge was reduced by over Example II The following compositions weremade up:

(1) Ferromanganese (approximately 20 grams), 10%

sodium silicate (binder).

(2) Mn0 (approximately 15 grams) coke (approximately 5 grams) and 10%sodium silicate (binder).

(3) MgO (approximately 15 grams) coke (approximately 5 grams) and 10%sodium silicate (binder).

These mixtures were rammed into holes diameter by 1 /2 in graphiteelectrodes, which were then baked for 2 hours at 200 C.

A first experiment was carried out on two control electrodes. This wasdone by striking and lengthening the arc. Owing to the arcs dying outthis procedure was repeated three times before sufficient heat wasobtained in the furnace. Marked fluctuations in the electrical loadranging from 1530 kva. were observed. The maximum length of arc was 7Following the introduction of the electrodes containing composition (1),a marked improvement was immediately observed. Considerable reduction innoise resulted and no fluctuations occurred, the furnace electrical loadbeing steady at 25 kva. By this method the length of the arc wasincreased immediately to 1%-l /z and held for 2 minutes, during whichperiod the evolution of brown fumes was observed.

On using the electrodes containing composition (2), the maximum lengthof arc obtainable in the test furnace, approximately 2", was immediatelyattained and held for 7 minutes, before switching the furnace off.Appreciable amounts of fumes were observed up to a period ofapproximately 5 minutes. Again considerable reduction in noise resultedand during the whole period the kva. reading remained steady at 25.

Electrodes containing composition (3) were then used. Again considerableimprovement resulted, the kva meter being steady at 22 and a maximum arclength of 2 was obtained immediately. Copious white fumes were observedfor a period of 7 minutes and the furnace left on for 15 minutes withoutany fluctuations occurring. After 15 minutes, the kva. reading hadincreased to 18.

While for the sake of succinctness the invention has been described onlywith relation to the method wherein the conductivity increasingsubstance is provided in or on an electrode, it will be understood thatit may alternatively be supplied by any other of the methods referred toearlier herein, with analogous results.

I claim as my invention:

1. In the melting of metal and in the production of metal frommetal-containing ore and a reducing agent, in an arc furnace, a methodfor the stabilization of an electric arc comprising forming the arebetween said metal and at least one electrode which is a carbonelectrode having a core including a proportion 'of a substance which,under the action of the arc, ionises to increase the electricalconductivity of the arc, the said substance being selected from thegroup consisting of lithium, barium, calcium, magnesium, chromium,manganese and silicon.

2. In the melting of metal and in the production of metal from themetal-containing ore and a reducing agent, in an arc furnace, a methodfor the stabilization of an electric are which comprises forming the arebetween said metal and at least one electrode which is a canbonelectrode having a core including a proportion of a substance which,under the action of the arc, ionises to increase the electricalconductivity of the arc, the said substance being selected from thegroup consisting of oxides, carbonates and silicates of lithium, barium,calcium, magnesium, chromium, manganese and silicon.

3. In the melting of metal and in the production of metal frommetal-containing ore and a reducing agent, in an arc furnace, a methodfor the stabilization of an electric arc which comprises forming the arebetween said metal and at least one electrode which is a carbonelectrode having a core including a proportion of a substance which,under the action of the arc, ionises to increase the electricalconductivity of the arc, the said substance being selected from thegroup consisting of compounds of lithium, sodium, potassium, rubidium,and caesium.

,4. A method as set forth in claim 3 wherein said substance is selectedfnom the group consisting of oxides, carbonates, and silicates oflithium, sodium, potassium, rubidium, and caesium.

5. The method of claim 11 wherein the said substance is present inparticulate form and the core is bonded With a binder selected from thegroup consisting of tar, pitch and a paste of anthracite, tar and pitch.

6. The method of claim 2 wherein the said substance is present inparticulate form and the core is bonded with a binder selected from thegroup consisting of tar, pitch and a paste of anthracite, tar and pitch.

7. The method of claim 3 wherein the said substance is present inparticulate form and the core is bonded With binder selected from thegroup consisting of tar, pitch and a paste of anthracite, tar and pitch.

8. The method of claim 3 wherein the said compound is a carbonate.

References Cited UNITED STATES PATENTS Ladoff 3l3354 X Bassett 3l3354XMott 313-354 X Dorcas 313354 X Parisot 313-654 Parisot 313-354 X Simonet a1 313354 X Parisot 313354 Wasserman et a1. 313-357 X Lauzau et a1.313-357 X 15 JOHN W. HUCKERT, Primary Examiner A. J. JAMES, AssistantExaminer US. Cl. X.R.

